1. Field of the Invention
The present invention relates to an optical transmitter and an optical transmission system, and in particular, to an optical transmitter and an optical transmission system that can improve transmission characteristics by suppressing non-linear effects occurring in an optical fiber transmission line.
2. Description of Related Art
In the field of optical communication, optical signals are conventionally transmitted via repeaters including 3R functions (retiming, reshaping, and regenerating). Such a repeater converts a transmitted optical signal into an electric signal, which is then subjected to predetermined processing and converted into the optical signal again before transmission to an optical transmission line. Recently developed optical fiber amplifiers, however, have enabled signals to be transmitted over a long distance of several thousands of km""s using only repeaters without the 3R functions. Consequently, the reliability of the optical transmission system has been improved and costs have been reduced. On the other hand, to further increase the transmission capacity, active efforts are being made to research a wavelength-division multiplexed (WDM) optical transmission system that transmits optical signals having mutually different wavelengths using a single transmission line. The wavelength-division multiplexed optical transmission has already realized a transmission capacity of larger than 1 Tb/s over a transmission distance of several hundred km. Such improved transmission distance and capacity have mostly been achieved by the ability of the optical fiber amplifier to amplify signal lights through a plurality of channels at a time.
Research is being widely conducted to implement long-distance and large-capacity optical transmissions using the optical fiber amplifier and WDM technology. The demand for the transmission capacity is expected to explosively increase due to the international spread of Internet. In particular, the demand for transmissions between Japan and the U.S. is predicted to increase, so the energetic research of very-long-distance (xcx9c10,000 km) systems across the Pacific Ocean is under way.
In order to realize long-distance transmissions, the waveform must be prevented from being degraded due to various factors. For example, the waveform may be degraded due to the polarization dependent effect occurring in the transmission line. The polarization dependent effect refers to the dependency of the transmission characteristics of optical fibers on the polarization state of signal light, and includes polarization dependent loss.
Various techniques have been developed to suppress the polarization dependent effect occurring in the optical fiber transmission line. For example, to suppress the polarization dependent effect occurring in the transmission line, bit-synchronized polarization scramble can be applied to signal light output from an optical transmitter. The bit-synchronized polarization scrambling can be applied for signal light in order to eliminate polarization effect of the transmission line. This technique smoothes the polarization dependent effect for each bit to prevent the transmission characteristics from varying depending on the polarization state of signal light.
There are, however, various other factors causing the waveform to be degraded during propagation through optical fibers, and appropriate measures are required for these factors.
It is an object of this invention to provide an optical transmitter and an optical transmission system that can sufficiently suppress waveform degradation caused by an SPM-GVD effect.
To achieve this object, an optical transmitter according to the present invention comprises an optical modulator for modulating the intensity of input light based on transmission data to output modulated light and a polarization modulator for modulating the polarization of the modulated light based on a polarization modulating signal, wherein each bit of the polarization modulating signal is set to have the same time interval as each bit of the transmission data.
Furthermore, the polarization modulator sets the polarization of each bit of the modulated light to one of two orthogonal polarizations for output, according to the polarization modulating signal.
The optical modulator may comprise an intensity modulator for modulating the intensity of input light based on the transmission data to output intensity-modulated light and a phase modulator for modulating the phase of the intensity-modulated light based on a bit rate frequency sine wave that is a sine wave of a frequency corresponding to the bit rate of the transmission data.
The optical modulator may comprise a phase modulator for modulating the phase of the intensity-modulated light based on a bit rate frequency sine wave that is a sine wave of a frequency corresponding to the bit rate of the transmission data to output phase-modulated light and an intensity modulator for modulating the intensity of the phase-modulated light based on the transmission data.